--- title: "Why do elements in a group behave alike?" subtitle: "Years 9 and 10 Chemistry · Atoms and the periodic table · Lesson 11" --- # Why do elements in a group behave alike? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Lithium, sodium and potassium look nothing alike in a jar, yet drop any of them in water and they fizz across the surface. Why do they behave the same? :::teacher-note **In one sentence:** Students explain why elements in the same group have similar chemical properties using outer-shell electrons, contrasting group 1, group 17 and the noble gases (AC9S10U06, node 2, a hinge node). **Students should be able to say:** I can explain that elements in the same group have the same number of outer-shell electrons. I can use outer electrons to explain the properties of a group. I can contrast a reactive group with the noble gases. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **Lithium, sodium and potassium all react with water in the same way. What do you predict their atoms have in common?** ☐ The same number of outer-shell electrons ☐ The same total number of electrons ☐ The same mass ☐ Full outer shells My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Each has one outer-shell electron: lithium 2, 1; sodium 2, 8, 1; potassium 2, 8, 8, 1. Elements in the same group have the same number of outer electrons, so they react in similar ways. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: outer electrons decide :::keep-together ### Sort by group **Which group does each belong to?** Groups: Group 1, Group 2, Group 17, Group 18 - Lithium (2, 1): {{field-15}} - Magnesium (2, 8, 2): {{field-15}} - Fluorine (2, 7): {{field-15}} - Chlorine (2, 8, 7): {{field-15}} - Neon (2, 8): {{field-15}} - Argon (2, 8, 8): {{field-15}} - Potassium (2, 8, 8, 1): {{field-15}} ::: :::teacher-note **Answer:** Lithium (2, 1) → Group 1; Magnesium (2, 8, 2) → Group 2; Fluorine (2, 7) → Group 17; Chlorine (2, 8, 7) → Group 17; Neon (2, 8) → Group 18; Argon (2, 8, 8) → Group 18; Potassium (2, 8, 8, 1) → Group 1 ::: :::keep-together ### Noble gases **Why do the noble gases (group 18) hardly react?** ☐ They have no electrons ☐ They are gases ☐ Their outer shells are full, so they don't tend to lose, gain or share electrons ::: :::teacher-note **Answer:** Their outer shells are full, so they don't tend to lose, gain or share electrons ::: :::keep-together ### Predict X **Element X is in group 17. What do you predict about it?** ☐ Seven outer electrons; it gains one easily, so it is a reactive non-metal ☐ A full outer shell; unreactive ☐ One outer electron; a reactive metal ::: :::teacher-note **Answer:** Seven outer electrons; it gains one easily, so it is a reactive non-metal ::: :::keep-together ### Sodium and chlorine **Why do sodium (group 1) and chlorine (group 17) react together so readily?** ☐ Sodium loses one electron easily and chlorine gains one easily ☐ They have the same number of outer electrons ☐ They are both metals ::: :::teacher-note **Answer:** Sodium loses one electron easily and chlorine gains one easily Each ends up with a full outer shell: sodium by losing one electron, chlorine by gaining one. They form sodium chloride (salt). ::: :::typical ## Elaborate: halogens and noble gases :::keep-together ### Helium **Helium has only 2 electrons, not 8. Why is it in group 18?** ☐ Because it is a gas, like the other noble gases ☐ Its only shell holds 2, so its outer shell is full, like the other noble gases ☐ It's a mistake in the table ::: :::teacher-note **Answer:** Its only shell holds 2, so its outer shell is full, like the other noble gases ::: ::: :::wider ## Elaborate: down the group :::keep-together **Potassium reacts with water much more violently than lithium. Why, when both have one outer electron?** ☐ Potassium's outer electron is further from the nucleus, so it is held less tightly and lost more easily ☐ Potassium has more outer electrons ☐ Potassium is heavier ::: :::teacher-note **Answer:** Potassium's outer electron is further from the nucleus, so it is held less tightly and lost more easily More shells means the outer electron is further out and more shielded by the inner electrons: easier to lose, so more reactive. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Elements in the same **group** have the same number of **outer-shell electrons**, so they react in similar ways. - **Group 1** (alkali metals): one outer electron, easily lost: very reactive metals. - **Group 17** (halogens): seven outer electrons, one short of full: very reactive non-metals. - **Group 18** (noble gases): full outer shells: hardly react. - The **period** (row) tells you how many shells an atom has. ## Exit ticket :::keep-together **Bromine is in group 17. Predict how many outer electrons it has and how it reacts. Explain.** {{lines-3}} ::: :::teacher-note **Answer:** Seven outer electrons; it gains one easily, so it is a reactive non-metal, like chlorine. :::